MAP vs DAP: choosing a starter fertiliser

What is in the bag

Per kilogram of phosphorus the two are close. The real difference is the nitrogen that comes with it. Figures below use Incitec Pivot's published analyses; other makers' MAP and DAP are similar but check each product page.

That extra N in DAP is counted in your nitrogen budget, not on top of it. See the nitrogen budget worked example.

Seed safety

Ammonium fertilisers can damage germinating seed through ammonia and through salt drawing water away from it. Incitec Pivot ranks the risk as urea, then DAP, then MAP, then ammonium sulphate, and describes MAP as safer for seed placement because it releases less ammonia.

• A Griffith University PhD study found DAP and urea cut establishment more than the same ammonium-N from MAP; crops usually tolerated 20–30% more ammonium-N from MAP than from DAP.
• Canola is the most sensitive winter crop. Wheat, barley and oats are more tolerant than lupins and chickpeas.
• Sandy soils and dry soil at or after sowing raise the risk. GRDC GroundCover notes damage happens when granules are not quickly diluted by soil moisture or rain.
• Narrow disc furrows and wider row spacing at the same rate concentrate fertiliser near the seed. Incitec Pivot suggests at least 50 mm separation, below and to the side.

Moisture matters a lot. In an Incitec Pivot trial at Young, NSW in 2022, with good soil moisture, up to 180 kg/ha of MAP (40 kg P) banded with canola seed did not reduce plant numbers. That is one manufacturer trial in a good season, not a safe rate for every paddock. Check the maker's seed-row rate tables for your crop, row spacing and soil.

Cost per kg of phosphorus

Cost per kg P = price per tonne ÷ (10 × P%). For MAP that is price ÷ 219, for DAP price ÷ 200. We hold no verified MAP or DAP price, so plug in your own quotes. A tonne of DAP carries 80 kg more N than a tonne of MAP (180 vs 100 kg) and 19 kg less P.

If you need that N, value it at what urea N costs you. At the $820/t Geelong urea listing ($1.78 per kg N), 80 kg of N is worth about $143. If you do not need the N, ignore it and compare on P alone. The cost per kg of nitrogen guide shows the same method.

Storage, blending and soil

• Incitec Pivot says MAP is less likely than DAP to absorb moisture and set in storage, and is preferred in blends stored before use.
• The same page says DAP should be applied shortly after blending, and is best used on acidic soils or where it is not in contact with the seed.
• Both are manufacturer statements; ask your agronomist how they apply to your soil.

How to choose

• Sowing canola or other sensitive crops with fertiliser in the seed row: MAP is the lower-risk option.
• Banding well away from the seed, broadcasting or blending for immediate use, and you want the extra N: DAP can make sense.
• Either way, set the P rate from a soil test. Read how to read a soil test for phosphorus tests and PBI.
• Compare zinc and sulphur-enhanced versions too, such as Granulock Z, if your agronomist says you need them.